1. Field of the Invention
The present invention generally relates to valve condition determination, and in particular to devices and methods for determining and monitoring the condition and operation of heart valves.
2. Description of the Prior Art
The human heart comprises four heart valves controlling the flow of blood from the atriums to the ventricles and from the ventricles further on into the pulmonary or systemic circulation system. The operation of the heart valves is critical for the well-being of the subject and any valve malfunctions may lead to severe and possibly life-threatening conditions.
Generally, blood flowing incorrectly backwards through a heart valve, i.e. regurgitation, is either a primary valve related problem that might cause acute heart failure or is a secondary problem in heart failure patients. If it is a primary valve related problem, i.e. the valve has ruptured or has been damaged, e.g. through infection, valve surgery is typically employed, where the damaged valve is repaired or replaced with a new artificial valve. Any heart failure will then often resolve automatically once the valve function has been restored.
If it is a secondary problem in heart failure patients, the main source for regurgitation is probably caused by the dilated state of the heart, making it difficult for the valve to close tightly. In this latter case, monitoring valve condition and status may serve as a valuable tool to monitor heart failure.
Another common heart valve problem is valve stenosis, where the valve kinetics are disturbed making it difficult to close properly or open sufficiently.
There is therefore a need for a tool of monitoring heart valve function in order to detect any deleterious heart valve effects and/or detect primary medical conditions manifesting in change in heart valve operation.
US 2007/0191901 discloses a cardiac resynchronization therapy (CRT) device that is being programmed based on various impedance-related parameters. Multi-vector impedance signals associated with dynamic intracardiac impedance are acquired and related to specific time frames of the cardiac cycle to derive indices representative of systolic and diastolic cardiac performance. The impedance signals are further adjusted by static impedance signals associated with pulmonary impedance as to derive composite indices representative of cardiac performance and pulmonary vascular congestion.
US 2007/0191901 also discusses that aortic valve stenosis can be detected using an aortic valve function:
  f  =      1                  T        AVO            -                        T          z                                      ⅆ            Z                                ⅆ            t                              where TAVO denotes the time of aortic valve opening, TZ denotes the onset time of positive impedance slope and
      ⅆ    Z        ⅆ    t  is the first derivative of the impedance signal and is included to account for cardiac output. A similar equation can be used for assessment of aortic valve regurgitation using delays in time to aortic valve closure from the onset of aortic valve opening or from time of peak impedance.